Apparatus and method for supporting handover with multi-connectivity in wireless communication system

ABSTRACT

The present disclosure relates to 5 th  generation (5G) or pre-5G communication systems for supporting higher data transfer rates, following the 4 th  generation (4G) communication systems, such as long term evolution (LTE). A method for supporting handover with multi-connectivity in a wireless communication system is provided. The method includes sending a measurement report message including information of at least one small base station (eNB) to a macro cell eNB, receiving information of a new small eNB in a target cell from the macro cell eNB based on the measurement report message, and performing a random access procedure with the new small eNB in the target cell based on handover.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of prior application Ser.No. 15/243,348, filed on Aug. 22, 2016, which was based on and claimedpriority under 35 U.S.C. § 119(a) of an Indian Provisional patentapplication filed on Aug. 21, 2015 in the Indian Intellectual PropertyOffice and assigned Serial number 4378/CHE/2015, of an IndianProvisional patent application filed on Aug. 21, 2015 in the IndianIntellectual Property Office and assigned Serial number 4402/CHE/2015,and of an Indian Provisional patent application filed on Mar. 31, 2016in the Indian Intellectual Property Office and assigned Serial number201641011346, the disclosure of each of which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to an apparatus and method for supportinghandover with multi-connectivity in wireless communication systems.

BACKGROUND

To meet the demand for wireless data traffic having increased sincedeployment of 4th-generation (4G) communication systems, efforts havebeen made to develop an improved 5th-generation (5G) or pre-5Gcommunication system. Therefore, the 5G or pre-5G communication systemis also called a ‘beyond 4G network’ or a ‘post long term evolution(LTE) system’.

The 5G communication system is considered to be implemented in higherfrequency (mmWave) bands, e.g., 28 GHz, 39 GHz, or 60 GHz bands, so asto accomplish higher data rates. To decrease propagation loss of theradio waves and increase the transmission distance, the beamforming,massive multiple-input multiple-output (MIMO), full dimensional MIMO(FD-MIMO), array antenna, an analog beam forming, large scale antennatechniques are discussed in 5G communication systems.

In addition, in 5G communication systems, development for system networkimprovement is under way based on advanced small cells, cloud radioaccess networks (RANs), ultra-dense networks, device-to-device (D2D)communication, wireless backhaul, moving network, cooperativecommunication, coordinated multi-points (CoMP), reception-endinterference cancellation and the like.

In the 5G system, hybrid frequency shift keying (FSK) and quadratureamplitude modulation (QAM) modulation (FQAM) and sliding windowsuperposition coding (SWSC) as an advanced coding modulation (ACM), andfilter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA),and sparse code multiple access (SCMA) as an advanced access technologyhave been developed.

In the meantime, high data rate service can be supported by cooperationof multiple base stations (BSs), for example, dual connectivity betweenmacro cell BS (MeNB) and small cell BS (SeNB) or between small cell BSsor between macro cell BSs. Existing dual connectivity scheme is hard tosupport ultra-high data rate service since addition of new small cell BSoperated in high frequency bands (e.g., >4 GHz or >3.5 GHz) causeson-going data degradation. More severe service degradation isexperienced in case of radio link failure at small cell BS operated inhigh frequency bands (e.g., >4 GHz or >3.5 GHz). Such BS cooperationschemes should be enhanced for continuous ultra-high data rate serviceor ultra-low latency service which requires high speed mobility and/orultra-high data rate and/or fast recovery from sudden signal drop and/orfast recovery from radio link failure.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

Aspects of the present disclosure are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide an apparatus and method for supportingcontinuous ultra-high rate data services that require fast recovery incase of radio link failure in a small cell base station (BS).

Another aspect of the present disclosure is to provide an apparatus andmethod for supporting multi-connectivity using multiple small cells.

In accordance with an aspect of the present disclosure, a method forsupporting handover with multi-connectivity in a wireless communicationsystem is provided. The method includes sending a measurement reportmessage including information of at least one small base station (eNB)to a macro cell eNB, receiving information of a new small eNB in atarget cell from the macro cell eNB based on the measurement reportmessage, and performing a random access procedure with the new small eNBin the target cell based on handover.

In accordance with another aspect of the present disclosure, anapparatus for supporting handover with multi-connectivity in a wirelesscommunication system is provided. The apparatus includes a transmitterfor sending a measurement report message including information of atleast one small eNB to a macro cell eNB, a receiver for receivinginformation of a new small eNB in a target cell from the macro cell eNBbased on the measurement report message, and a controller for performinga random access procedure with the new small eNB in the target cellbased on handover.

In accordance with another aspect of the present disclosure, a methodfor supporting handover with multi-connectivity in a wirelesscommunication system is provided. The method includes receiving ameasurement report message including information of at least one smalleNB from a user equipment (UE), and sending information of a new smalleNB in a target cell to the UE based on the measurement report message,wherein a random access procedure is performed based on handover betweenthe UE and the new small eNB in the target cell.

In accordance with another aspect of the present disclosure, anapparatus for supporting handover with multi-connectivity in a wirelesscommunication system is provided. The apparatus includes a receiver forreceiving a measurement report message including information of at leastone small eNB from a UE, and a transmitter for sending information of anew small eNB in a target cell to the UE based on the measurement reportmessage, wherein a random access procedure is performed based onhandover between the UE and the new small eNB in the target cell.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses various embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present disclosure will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a wireless communication system accordingto an embodiment of the present disclosure;

FIG. 2 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure;

FIG. 3 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure;

FIGS. 4A, 4B, and 4C illustrate how to obtain small cell base station(SeNB) information according to an embodiment of the present disclosure;

FIG. 5 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toan embodiment of the present disclosure;

FIG. 6 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure;

FIG. 7 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure;

FIG. 8 is a block diagram of a wireless communication system accordingto a second embodiment of the present disclosure;

FIG. 9 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIG. 10 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIG. 11 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIG. 12 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIG. 13 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIG. 14 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure;

FIGS. 15A and 15B is a block diagram of a wireless communication systemaccording to a third embodiment of the present disclosure;

FIG. 16 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure;

FIG. 17 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure;

FIG. 18 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure;

FIGS. 19, 20, 21, 22, 23, 24, 25, and 26 are flowcharts illustratingprocedures of SeNB addition according to various embodiments of thepresent disclosure;

FIG. 20 presents macro cell base station (MeNB) initiated SeNB selectionwith sequential attachment according to an embodiment of the presentdisclosure;

FIG. 21 presents user equipment (UE) initiated SeNB selection withsimultaneous attachment according to an embodiment of the presentdisclosure;

FIG. 22 presents UE initiated SeNB selection with sequential attachmentaccording to an embodiment of the present disclosure;

FIG. 23 presents MeNB initiated multiple primary cell (PCell) setconfiguration, according to an embodiment of the present disclosure;

FIG. 24 presents MeNB initiated multiple primary secondary cell (PSCell)set configuration according to an embodiment of the present disclosure;

FIG. 25 presents MeNB initiated multiple PCell set configuration duringSeNB change according to an embodiment of the present disclosure;

FIG. 26 presents MeNB initiated multiple PCell set and PSCell setconfiguration during MeNB change according to an embodiment of thepresent disclosure;

FIG. 27 is a flowchart of operation of UE in a wireless communicationsystem according to an embodiment of the present disclosure;

FIG. 28 is a flowchart of operation of MeNB in a wireless communicationsystem according to an embodiment of the present disclosure;

FIG. 29 is a flowchart of operation of SeNB in a wireless communicationsystem according to an embodiment of the present disclosure;

FIG. 30 is a block diagram of a UE in a wireless communication systemaccording to an embodiment of the present disclosure;

FIG. 31 is a block diagram of an MeNB in a wireless communication systemaccording to an embodiment of the present disclosure;

FIG. 32 is a block diagram of an SeNB in a wireless communication systemaccording to an embodiment of the present disclosure.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of variousembodiments of the present disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the various embodiments describedherein can be made without departing from the scope and spirit of thepresent disclosure. In addition, descriptions of well-known functionsand constructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of the presentdisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of various embodiments of the presentdisclosure is provided for illustration purpose only and not for thepurpose of limiting the present disclosure as defined by the appendedclaims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

User equipment (UE) or communication terminal is an entity forcommunicating with a base station or other terminals and may also bereferred to as node, UE, mobile station (MS), mobile equipment (ME),device, terminal, etc. Hereinafter, a value of TTT in FIGS. 2, 3, 5, 6,7, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, and26, is set to ‘0’ or more.

Hereinafter, macro cell base station (BS) and macro cell BS (MeNB) areinterchangeably used, and small cell BS and small cell BS (SeNB) arealso interchangeably used. It should be noted that the interchangeablyused terms have the same meaning. In this disclosure, small cell BSoperates on long term evolution (LTE) frequency or mmWave frequency orhigher frequency than LTE or lower frequency than LTE. Small cell BSoperates in licensed band or unlicensed band. Small cell BS can be abase station with cellular technology or LTE technology or Wi-Fitechnology or WiGig technology. Small cell BS has the same capacity asmacro cell BS but small cell BS performs some part of normal basestation functions (e.g., no radio resource control (RRC) function, userpacket transmission and reception) as SeNB according to this disclosure.As another example small cell BS has less capacity than macro cell BS;small cell BS is a transmission point, or small cell BS has layer 1functions only, or small cell BS has layer 1 and MAC sub-layer functionsonly etc. As one example, small cell BS has connection to a gateway(e.g., S-GW). In other example small cell BS does not have connection toa gateway (e.g., S-GW) and is connected to a gateway through macro cellBS.

In this disclosure, macro cell BS indicates a base station whichoperates in current LTE system.

A metric to perform new SeNB addition during MeNB handover is signalquality of SeNB of Target MeNB. A metric to perform new SeNB additionduring MeNB handover is available resource at SeNB with which UE getcontinuous ultra-high data rate packet transmission/reception at TargetMeNB. A metric to perform new SeNB selection is signal quality of SeNBor available resource at SeNB with which UE get continuous ultra-highdata rate packet transmission/reception when UE is connected to the newSeNB.

FIG. 1 is a block diagram of a wireless communication system accordingto an embodiment of the present disclosure.

Referring to FIG. 1, UE 100 is simultaneously or sequentially served bysmall cell BS (SeNB) 110 as well as macro cell BS (MeNB) 120, where MeNB120 refers to Serving-MeNB (S-MeNB) and SeNB 110 refers to Serving-SeNB(S-SeNB).

In an embodiment, in dual connectivity the two BSs may transmit/receivepackets to/from UE. The packets served with the two BSs can be from thesame data bearer or different data bearers. UE 100 gets measurementconfiguration information including a metric from S-MeNB 120 andperforms measurement on other BSs (other MeNBs and other SeNBs). Metricand measurement may be applied to at least one of beam indexused/measured between UE 100 and S-MeNB 120, between UE 100 and S-SeNB110, between UE 100 and other MeNB(s) 140, between UE 100 and otherSeNB(s) 130. A metric applied to SeNB 110 for SeNB addition can bedifferent that for MeNB 120. UE 100 reports measurement result ofSeNB(s) 110 as well as MeNB(s) 120, where an identifier of SeNB(s)and/or beam state information, beam index are reported. Based on ameasurement result of UE 100 the S-MeNB 120 may select other SeNB whichcan serve UE as new SeNB 140 under the S-MeNB. Once an event when S-SeNBcannot serve UE (e.g., a signal quality of S-SeNB drops below a certainthreshold 1) happens the newly selected SeNB may become new S-SeNB forUE.

In another embodiment, based on a measurement result of UE 100 theS-MeNB 120 may select candidate new MeNB where UE 100 performs handoveraccording to a predefined event condition (e.g., a signal quality ofS-MeNB drops below a certain threshold 2). During UE's handovertransaction 150 between S-MeNB and candidate new MeNB the S-MeNB mayshare UE's measurement result of SeNB(s) under the new MeNB. UE'smeasurement result of SeNB(s) may include at least one of measuredsignal quality, measured beam state information, beam index. With themeasurement result the new MeNB may select new SeNB which can serve UE100 with the new MeNB. The selected new SeNB of new MeNB may be informedto UE through S-MeNB during UE's 100 handover transaction. Dedicatedrandom access resource at the selected new SeNB or the selected new MeNBcan be provided for UE 100 based on beam state information, beam indexmeasured by UE 100. UE context maintenance at the selected new SeNB 130or the selected new MeNB 140 can be provided for UE 100, where UEcontext may be kept after UE access to the selected new MeNB 140 or theselected new SeNB 130. Once completing connection reconfigurationprocedure with the new MeNB, UE 100 may resume packettransmission/reception with the new MeNB 140 and the new SeNB 130.

FIG. 2 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure.

Referring to FIG. 2, in operation 201, S-MeNB transmits measurementcontrol message to UE where the measurement control message includesmeasurement control information, which may include at least one of otherMeNB information, SeNB information for example, center frequency,primary cell identification (PCell ID), BS ID, measurement eventcondition, measurement report event condition, etc. Measurement eventcondition and/or measurement report event condition for SeNB additioncan be different from that for MeNB handover. According to measurementcontrol information UE measures MeNBs as well as SeNB periodically orevent-based, in operation 203. UE reports its measurement result toS-MeNB, in operation 205. The measurement result may include at leastone of SeNBs' signal quality under S-MeNB, other MeNBs' signal quality,or SeNBs' signal quality under other MeNBs or beam state information,beam index. Based on the measurement result S-MeNB determines whether UEneeds to perform handover to another MeNB, in operation 207. If UE'shandover is required then S-MeNB transmits HO request message tocandidate new MeNB, in operation 209. The HO request message may includeat least one of UE's measurement result of SeNBs under the candidate newMeNB including beam state information, beam index for SeNBs as well asUE's information. If candidate new MeNB accepts UE's handover to itselfthen the candidate new MeNB selects SeNB for the UE, in operation 211.The candidate new MeNB transmits SeNB Add request message to theselected SeNB, in operation 213. The SeNB responds with SeNB Addresponse message to the candidate new MeNB, in operation 215. Thecandidate new MeNB transmits HO request ack message to the S-MeNB, inoperation 217, where at least one of UE's HO acceptance, SeNBinformation, RACH information may be included in the HO request ackmessage. RACH information at SeNB may be selected based on at least oneof beam state information, beam index for SeNB. The S-MeNB transmits HOCommand message to UE, in operation 219, where the informationtransmitted from the candidate new MeNB may be included in the HOCommand message, e.g. at least one of MeNB information, SeNBinformation, RACH information etc.

The S-MeNB releases the connection between the S-MeNB and the S-SeNB inoperation 221, after or at the same time it sends the HO Command messageto the UE. The candidate new MeNB becomes target MeNB (T-MeNB). TheS-MeNB communicates data with the T-MeNB, in operation 223. UE beginsconnection reconfiguration procedure with the T-MeNB in operations 221and 223. In connection reconfiguration procedure, UE perform randomaccess to T-MeNB, in operation 225. In an embodiment, UE may performrandom access to new SeNB. The T-MeNB then sends an SeNB additioncomplete message to the new SeNB, in operation 227. In connectionreconfiguration procedure of the UE, S-MeNB, S-SeNB, T-MeNB and new SeNBmay perform path change procedures for UE through X2 interface or Siinterface, as in operation 223. Furthermore, in connectionreconfiguration procedure of the UE, an SeNB release procedure betweenS-MeNB and S-SeNB is performed as in operation 221. Specifically, inhandover transaction and connection reconfiguration of the UE with theT-MeNB, the SeNB and T-MeNB may perform path setup and packet deliveryprocedures for continuity of services to the UE.

Pending UE's packets at S-MeNB and S-SeNB may be transferred to T-MeNB.In the connection reconfiguration procedure, the UE performs randomaccess to the new SeNB, in operation 229, and data is forwarded inoperation 231 as illustrated in FIG. 2.

FIG. 3 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure.

Referring to FIG. 3, S-MeNB transmits measurement control message to UE,in operation 301. Measurement control message includes measurementcontrol information, which may include at least one of measurementcontrol of candidate target MeNBs, candidate target SeNBs, measurementreport triggering condition for MeNB as well as SeNB. Measurement eventcondition and/or measurement report event condition for SeNB additioncan be different from that for MeNB handover. UE performs measurementaccording to the information of measurement control message, inoperation 303. In operation 305, if an event indicated in measurementcontrol message happens, then UE waits for the expiration of TTT(time-to-trigger). A value of TTT is set to ‘0’ or more. The event ofmeasurement report continues at the expiration of TTT in operation 307,then UE transmits measurement report message to S-MeNB. When the eventrequires measurement report of candidate target MeNB, UE reports SeNBsunder candidate target MeNB (e.g., SeNB's center frequency and SeNB ID).The S-MeNB transmits HO request message to candidate target MeNB, inoperation 309. The HO request message may include measurement result(e.g., at least one of RSRP, RSRQ, SINR, beam state information, andbeam index) of SeNBs under candidate target MeNB, where the measurementis performed on beam index of SeNBs. The HO request message may includeat least one candidate SeNB identifier under candidate target MeNB.Based on the SeNB information of HO request message, candidate targetMeNB selects candidate SeNB for UE in operation 311 and sends SeNB addrequest message to the candidate SeNB in operation 313. The candidateSeNB responds with SeNB add request ack message, in operation 315. TheSeNB add request ack message may include at least one of RACHinformation of SeNB, dedicated RACH preamble, beam-forming information(beam state information, beam index) of SeNB. The candidate target MeNBtransmits HO request ack message to S-MeNB, in operation 317 and the HOrequest ack message may include at least one of RACH information ofcandidate target MeNB, RACH information of SeNB, dedicated RACH preambleat candidate target MeNB, dedicated RACH preamble at SeNB, beamforminginformation of SeNB.

Referring to FIG. 3, in operation 321, S-MeNB may transmit RRCConnection Reconfiguration message to UE to command UE to performhandover to the target MeNB. The RRC Connection Reconfiguration messagemay include at least one of RACH information of target MeNB, RACHinformation of SeNB, dedicated RACH preamble at target MeNB, dedicatedRACH preamble at SeNB, beamforming information of SeNB. UE performsrandom access with the target MeNB using the information gotten in RRCConnection Reconfiguration message, in operation 331. UE completesrandom access with the target MeNB and transmits RRC ConnectionReconfiguration complete message to the target MeNB in operation 333. UEperforms random access with the new SeNB in operation 337 using theinformation gotten in RRC Connection Reconfiguration message. The UE canperform random access with T-MeNB and SeNB simultaneously.

While UE's handover transaction including connection reconfigurationwith T-MeNB and random access with new SeNB, S-MeNB, S-SeNB, T-MeNB andnew SeNB performs procedures which change path from S-MeNB and SeNB toT-MeNB and new SeNB in operations 319, 323, 325, 327, 329, 335, 339, and341, forward pending UE packets from S-MeNB and SeNB to T-MeNB and newSeNB. In other words, during handover transaction and connectionreconfiguration of the UE to the T-MeNB, the SeNB and T-MeNB may performpath setup and packet delivery procedures for continuity of services tothe UE.

FIGS. 4A, 4B, and 4C illustrate how to obtain SeNB information accordingto an embodiment of the present disclosure.

Referring to FIG. 4A, as in operation 401, MeNBs exchanges its own SeNBsinformation. The SeNB information may include at least one of SeNB'scenter frequency, SeNB's ID (globally unique ID), SeNB's PCell ID, etc.This information is not shared with UE, so UE may not know which SeNBbelongs to which MeNB.

Referring to FIG. 4B, as in operation 403, MeNBs (e.g., MeNB 1, MeNB 2)exchanges its own SeNBs information which is included in measurementcontrol message to UE. The SeNB information may include at least one ofSeNB's center frequency, SeNB's ID (globally unique ID), SeNB's PCellID, etc. In operation 405, UE receives measurement control messagegenerated in operation 403.

Referring to FIG. 4C, as in operation 407, UE sends measurement resultmessage to a serving MeNB, where measurement result message may includeat least one of SeNBs information and neighboring MeNBs information. TheSeNB information may include at least one of SeNB's center frequency,SeNB's ID (globally unique ID), SeNB's PCell ID, SeNB's beam stateinformation, beam index etc. After receiving measurement result, UE'sMeNB exchanges the information of measurement result message with otherMeNBs or central controller (e.g., MME), in operation 409. From thistransaction MeNB knows which SeNB belong to which MeNB. In anembodiment, based on measurement result from UE, MeNB or SeNB store therelationship of beam state information, beam index and SeNB which isused for deciding SeNB to add.

FIG. 5 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toan embodiment of the present disclosure.

Referring to FIG. 5, in operation 501, S-MeNB transmits measurementcontrol message to UE which includes at least one of candidate newMeNBs, candidate new SeNBs, measurement report control, measurementreport triggering condition etc. Measurement report control ormeasurement report triggering condition for SeNB addition is differentfrom those for MeNB handover. In operation 503, UE performs measurementbased on the measurement control information of measurement controlmessage. In operation 505, when an event is triggered according to themeasurement control information, UE waits for an expiration oftime-to-trigger (TTT) for MeNB handover. A value of TTT is set to ‘0’ ormore. In operation 507, UE transmits Measurement report message toS-MeNB. When S-MeNB determines UE's handover to candidate target MeNBbased on the measurement report in operation 509, S-MeNB transmits HOrequest message to the candidate target MeNB in operation 511. The HOrequest message may include at least one of UE's information, UE'smeasurement result of SeNB and SeNBs under the MeNB. In operation 513,the candidate target MeNB selects candidate new SeNB based on themeasurement result of SeNBs including at least one information of signalquality, beam state information, beam index, resource availability,stored information of the relationship between beam state information,beam index and SeNB. In operation 515, the candidate target MeNB andcandidate new SeNB exchange SeNB addition request and SeNB additionresponse messages. The SeNB addition request and SeNB addition messagesmay include at least one of UE information, RACH information of SeNB,dedicated RACH preamble, beam-forming information (beam stateinformation, beam index) of SeNB. In operation 517, the candidate targetMeNB transmits HO request ack message to S-MeNB that includes the randomaccess information for UE which performs handover to candidate targetMeNB and new SeNB. Random access information at new SeNB may bedetermined based on UE's beam state information, beam index report fornew SeNB. In operation 519, S-MeNB transmits RRC ConnectionReconfiguration message to UE with the information of HO request ackmessage. Using the random access information of RRC ConnectionReconfiguration message UE performs random access with T-MeNB and newSeNB, in operations 527, 529. After completing random access procedureswith T-MeNB and new SeNB, in operation 530, UE transmits RRC ConnectionReconfiguration Complete message to T-MeNB. In operations 521, 523, and525, UE performing handover transaction with S-MeNB and random accessprocedures with T-MeNB and new SeNB, S-MeNB, S-SeNB, T-MeNB and new SeNBperforms path change, pending packet forwarding procedures for UE. Inother words, in handover transaction and connection reconfiguration ofthe UE to the T-MeNB, the SeNB and T-MeNB may perform path setup andpacket delivery procedures for continuity of services to the UE.

FIG. 6 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure.

Referring to FIG. 6, in operation 601, S-MeNB transmits measurementcontrol message to UE. Measurement control message includes at least oneof measurement control of candidate target MeNBs, candidate targetSeNBs, measurement report triggering condition for MeNB as well as SeNB.Measurement report control or measurement report triggering conditionfor SeNB addition is different from those for MeNB handover. Inoperation 603, UE performs measurement according to the information ofmeasurement control message. In operation 605, if an event indicated inmeasurement control message happens, then UE waits for the expiration ofTTT (time-to-trigger) of MeNB handover. A value of TTT is set to ‘0’ ormore. The event of measurement report to T-MeNB continues at theexpiration of TTT, then UE transmits measurement report message toT-MeNB in operation 609. Before transmitting measurement report message,UE may perform random access with T-MeNB in operation 607. T-MeNBallocates UL grant for measurement report signaling when receivingrandom access from UE. The measurement report message uses predefinedrandom access resource as an indication of MeNB handover to T-MeNB. UEtransmits random access for measurement report instead of general randomaccess, in order for T-MeNB to allocate required random access resourcesto the measurement report message. In an additional embodiment, themeasurement report message may include measurement result (e.g., atleast one or more of RSRP, RSRQ, SINR, beam state information, beamindex) of SeNBs under T-MeNB. In another additional embodiment, themeasurement report message may include candidate SeNB identifier underT-MeNB. In operation 611, the T-MeNB transmits HO request message toS-MeNB which indicates that UE will handover to T-MeNB. HO requestmessage can include random access resource at T-MeNB and/or candidateSeNB. S-MeNB responds with HO Request ack message which includes UE'sinformation. In operation 613, the HO request and HO request ackmessages are used to fetch UE's context from S-MeNB to T-MeNB. Whenreceiving HO request message, S-MeNB performs SeNB release procedurewith S-SeNB in operation 615. In operation 619, T-MeNB performs SeNBaddition procedure with new SeNB through SeNB addition request and SeNBaddition response messages. In SeNB addition response message at leastone of RACH information of new SeNB, dedicated RACH preamble of new SeNBmay be included. In operation 619, T-MeNB transmits RRC ConnectionReconfiguration message to UE which includes at least one or more of newSeNB information (center frequency, identifier), RACH information of newSeNB, beamforming information of new SeNB, dedicated RACH preamble ofnew SeNB. UE responds with RRC Connection Reconfiguration Completemessage in operation 621, and performs random access procedure with newSeNB in operation 623. Although not shown, S-MeNB, S-SeNB, T-MeNB andnew SeNB perform path change procedures, UE packet forwarding after HOrequest/HO request ack transaction between S-MeNB and T-MeNB. Inoperation 617, T-MeNB performs SeNB addition procedure with new SeNBthrough SeNB addition request and SeNB addition response messages.

FIG. 7 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa first embodiment of the present disclosure.

Referring to FIG. 7, in operation 701, S-MeNB transmits measurementcontrol message to UE. Measurement control message may include at leastone of measurement control of candidate target MeNBs, candidate targetSeNBs, measurement report triggering condition for MeNB as well as SeNB.Measurement report control or measurement report triggering conditionfor SeNB addition is different from those for MeNB handover. Inoperation 703, UE performs measurement according to the information ofmeasurement control message. In operation 705, if an event indicated inmeasurement control message happens, then UE waits for the expiration ofTTT (time-to-trigger) of MeNB handover. A value of TTT is set to ‘0’ ormore. In operation 709, the event of measurement report to T-MeNBcontinues at the expiration of TTT, then UE transmits measurement reportmessage to T-MeNB. Before transmitting measurement report message, UEmay perform random access with T-MeNB in operation 707. T-MeNB allocatesUL grant for measurement report signaling when receiving random accessfrom UE. The measurement report message uses predefined random accessresource as an indication of MeNB handover to T-MeNB. The random accessresources for being allocated UL grant for the measurement reportmessage is distinguished from general random access resources. Themeasurement report message includes measurement result (e.g., at leastone of RSRP, RSRQ, SINR, beam state information, beam index) of SeNBsunder T-MeNB. The measurement report message may include candidate SeNBidentifier under T-MeNB. T-MeNB may determine whether UE can be servedby T-MeNB. T-MeNB determines whether SeNB is needed for serving UE basedon at least one of signal quality, beam state information, beam index,stored information of the relationship between beam state information,beam index and SeNB based on UE's report. In operation 711, T-MeNBperforms SeNB addition procedure with new SeNB through SeNB additionrequest and SeNB addition response messages. In SeNB addition responsemessage at least one of RACH information of new SeNB, dedicated RACHpreamble of new SeNB may be included. The RACH resource is determinedbased on at least one of beam state information, beam index of new SeNB.In operation 713, T-MeNB transmits RRC Connection Reconfigurationmessage to UE which includes new SeNB information (center frequency,identifier), RACH information of new SeNB, beamforming information (beamstate information, beam index) of new SeNB, dedicated RACH preamble ofnew SeNB based on at least one of beam state information, beam indexreported by UE. UE responds with RRC Connection Reconfiguration Completemessage in operation 715, and performs random access procedure with newSeNB in operation 717. T-MeNB may fetch UE's context from MME or T-MeNBmay get UE's context from S-MeNB.

In UE performs handover transaction with T-MeNB and/or accesses to SeNBof T-MeNB, T-MeNB may inform UE's handover to S-MeNB. Although not shownin FIG. 7, when receiving UE's handover notification, S-MeNB performsSeNB release procedure with S-SeNB. Although not shown in FIG. 7,S-MeNB, S-SeNB, T-MeNB and new SeNB perform path change procedures, UEpacket forwarding between S-MeNB and T-MeNB. In other words, duringhandover transaction and connection reconfiguration of the UE to theT-MeNB, the SeNB and T-MeNB may perform path setup and packet deliveryprocedures for continuity of services to the UE.

FIG. 8 is a block diagram of a wireless communication system accordingto a second embodiment of the present disclosure.

Referring to FIG. 8, there is a case where UE 810 should performshandover from serving macro cell BS 820 to target macro cell BS 840 buthandover condition of serving small cell BS 830 to new small cell is notmet (e.g., signal quality of serving small cell BS is good enough toserve UE 810. For example, a value of signal quality of serving smallcell BS is greater than a threshold). Such small cell BS 830 can serveUE 810 with multiple MeNBs, i.e., one small cell BS 830 operates underserving MeNB as well as target MeNB from UE 810 perspective.

Based on UE's 810 measurement report, S-MeNB 820 may determine UE's 810handover to T-MeNB 840. If UE's 810 handover condition to T-MeNB 840 ismet but SeNB 830 should be kept for UE 810 then the S-MeNB 820 informsUE's 810 handover with SeNB information to T-MeNB 840. T-MeNB 840 andSeNB 830 setup path for UE packet forwarding. UE's 810 pending packetsare forwarded from SeNB to T-MeNB 840 or from S-MeNB 820 to T-MeNB 840.While UE's handover transaction to T-MeNB 840 and UE's connectionreconfiguration with T-MeNB 840, SeNB and T-MeNB 840 can perform pathsetup and packet forwarding procedures for UE's service continuity.

FIG. 8 presents a case in which one SeNB serves UE at S-MeNB 820 as wellas T-MeNB 840. In an embodiment, more than 1 SeNB serving UE at S-MeNBcontinues to serve UE at T-MeNB 840. In detail, all SeNBs serving UE atS-MeNB 820 continue to serve UE at T-MeNB 840. Or some SeNBs serving UEat S-MeNB 820 continue to serve UE at T-MeNB 840 and other SeNBs servingUE at S-MeNB 820 stop serving UE at T-MeNB 840. Or some SeNBs serving UEat S-MeNB 820 continue to serve UE at T-MeNB 840 and new SeNBs are addedto serve UE at T-MeNB 840.

FIG. 9 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 9, in operation 901, S-MeNB transmits measurementcontrol message to UE. Measurement control message includes measurementcontrol of candidate target MeNBs, candidate target SeNBs, measurementreport triggering condition for MeNB as well as SeNB. Measurement reportcontrol or measurement report triggering condition for SeNB addition isdifferent from those for MeNB handover. In operation 903, UE performsmeasurement according to the information of measurement control message.In operation 905, if an event indicated in measurement control messagehappens, then UE waits for the expiration of TTT (time-to-trigger) ofMeNB handover. A value of TTT is set to ‘0’ or more. The event ofmeasurement report continues at the expiration of TTT, then UE transmitsmeasurement report message to S-MeNB in operation 907. When the eventrequires measurement report of candidate target MeNB, UE reports SeNBsunder candidate target MeNB (e.g., at least one of SeNB's centerfrequency, SeNB ID, beam state information, beam index of SeNB) toS-MeNB. According to UE's measurement report, S-MeNB determinescandidate target MeNB and candidate new SeNB in operation 908. If S-MeNBdetermines that serving SeNB should serve UE in candidate target MeNB,then the S-MeNB informs UE's handover to the candidate target MeNB andthe SeNB through HO request message in operation 909. The HO requestmessage includes SeNB information. If more than one SeNB serving UE maycontinue to serve UE at the candidate target MeNB, then the SeNBs areinformed to the candidate target MeNB. SeNBs beam state information,beam index is informed to the candidate target MeNB. Relationshipbetween SeNBs beam state information, beam index and SeNB is stored atthe candidate target MeNB which is used for deciding SeNB(s) to add. Inan additional embodiment, the HO request message may include measurementresult (e.g., at least one of RSRP, RSRQ, SINR, beam state information,beam index) of other SeNBs of the candidate target MeNB. In anotherembodiment, the HO request message may include other SeNBs' identifiersof the candidate target MeNB. The other SeNB information such as atleast one of signal quality, resource availability, beam stateinformation, beam index may be used for candidate target MeNB to checkwhether other SeNB can be a new SeNB instead of serving SeNB. Based onthe SeNB information of HO request message, candidate target MeNBdetermines serving SeNB for UE after handing-over to the candidatetarget MeNB in operation 910, where serving SeNB may be the same SeNB ofS-MeNB or is new SeNB. In operation 911, the candidate target MeNBresponds with HO Request ack message which includes at least one ofselected SeNB (serving SeNB or new SeNB), random access information ofcandidate target MeNB, dedicated RACH preamble of candidate target MeNB.When current SeNB is selected, then in operation 913, S-MeNB transmitsMeNB Change req message to the SeNB, and informs new target MeNBinformation. In operation 915, the SeNB responds with MeNB Change RSPmessage. The MeNB Change RSP message may include UE's packet informatione.g., Sequence Number (SN) status transfer.

Subsequently, in operation 917, S-MeNB sends an RRC connectionreconfiguration message to UE to perform handover to a target MeNB. TheRRC connection reconfiguration message may include at least one of RACHinformation of the target MeNB, dedicated RACH preamble at the targetMeNB, SeNB information (identifier of SeNB or indicator of change inSeNB). In operation 918, SN Status transfer occurs, as illustrated inFIG. 9.

In operation 919, UE uses information obtained from the RRC connectionreconfiguration message to perform random access with the target MeNB.After completion of the random access with the target MeNB, the UE sendsan RRC connection reconfiguration complete message to the target MeNB inoperation 921.

The UE may then perform operation of adding a new SeNB in operation 923.

FIG. 10 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 10, in operations 1003 and 1005, T-MeNB informs SeNBthat SeNB will serve UE with T-MeNB through SeNB addition procedurebetween T-MeNB and SeNB.

When data path has not setup between SeNB and T-MeNB then path setupprocedure is performed.

In operation 1017, S-MeNB transmits RRC Connection Reconfigurationmessage to command to perform handover to the target MeNB. The RRCConnection Reconfiguration message may include at least one of RACHinformation of target MeNB, dedicated RACH preamble at target MeNB, SeNBinformation (SeNB identifier or SeNB change indicator).

In operation 1021, UE performs random access with the target MeNB usingthe information gotten in RRC connection reconfiguration message. UEcompletes random access with the target MeNB and transmits RRCConnection Reconfiguration complete message to the target MeNB inoperation 1023. If SeNB at the target MeNB is same as those of S-MeNB,then random access with SeNB may be omitted.

While UE's handover transaction or connection reconfiguration with thetarget MeNB, if path between the target MeNB and serving SeNB has notbeen established then path setup procedure between the target MeNB andserving SeNB is performed. Also UE's pending packets can be forwardedbetween S-MeNB and T-MeNB or between serving SeNB and T-MeNB.

Based on UE's measurement for SeNB at the T-MeNB, new SeNB is selectedfor UE using SeNB addition procedure.

When SeNB no longer serve UE at T-MeNB then T-MeNB informs that SeNBrelease UE's contexts through SeNB release procedure.

FIG. 11 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 11, in operations 1103, 1105, during MeNB handoverprocedure, new SeNB addition procedure at target MeNB can be performedwhile a procedure to maintain serving SeNB at target MeNB is performed.The new SeNB information and no change of serving SeNB indication areprovided to UE through a signaling of handover command such as RRCconnection reconfiguration.

FIG. 12 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 12, in operation 1201, S-MeNB transmits measurementcontrol message to UE. Measurement control message includes measurementcontrol of candidate target MeNBs, candidate target SeNBs, measurementreport triggering condition for MeNB as well as SeNB. Measurement reportcontrol or measurement report triggering condition for SeNB addition isdifferent from those for MeNB handover. In operation 1203, UE performsmeasurement according to the information of measurement control message.In operation 1205, if an event indicated in measurement control messagehappens, then UE waits for the expiration of TTT (time-to-trigger) ofMeNB handover. A value of TTT is set to ‘0’ or more. The event ofmeasurement report to T-MeNB continues at the expiration of TTT, then UEtransmits measurement report message to the T-MeNB. Before transmittingmeasurement report message, UE may perform random access with T-MeNB inoperation 1207.

UE sends the measurement report message to the T-MeNB in operation 1209.T-MeNB allocates UL grant for UE to transmit measurement report inoperation 1209. A predefined random access resource can be used torequest UL grant for measurement report signaling. The random accessresources for being allocated UL grant for the measurement reportmessage is distinguished from general random access resources. Themeasurement report message includes measurement result (e.g., RSRP,RSRQ, SINR, beam state information, beam index) of serving SeNB,measurement result of other SeNBs under T-MeNB. The measurement reportmessage may include SeNBs' identifiers of T-MeNB. Based on themeasurement report (signal quality, beam state information, beam index)or resource availability at SeNB(s) the T-MeNB determines SeNB who serveUE after UE's handing-over to T-MeNB in operation 1210, where SeNB isthe same SeNB at S-MeNB or new SeNB. More than 1 SeNB serving UE atS-MeNB may continue serve UE at T-MeNB.

In operation 1211, the T-MeNB transmits HO request message to S-MeNBwhich indicates that UE will handover to T-MeNB and SeNB who serve UE atT-MeNB. In operation 1213, S-MeNB sends the T-MeNB HO Request ackmessage which may include UE's information.

T-MeNB then sends an RRC connection reconfiguration message to UE, inoperation 1205.

In operation 1207, S-MeNB transmits MeNB Change Req message to servingSeNB which includes T-MeNB information. In operation 1209, Serving SeNBresponds with MeNB Change RSP message which includes UE's packetforwarding information e.g., SN Status transfer. UE then sends an RRCconnection reconfiguration complete message to T-MeNB in operation 1217.UE may then perform operation of adding a new SeNB in operation 1213. Inoperation 1219, SN Status transfer occurs, as illustrated in FIG. 12.

FIG. 13 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 13, in an embodiment, in operations 1301, 1303, T-MeNBinforms SeNB that SeNB will serve UE with T-MeNB through SeNB additionprocedure between T-MeNB and SeNB.

If path has not been setup between T-MeNB and serving SeNB then pathsetup procedure between T-MeNB and serving SeNB is performed. Also UE'spending packets can be forwarded between S-MeNB and T-MeNB or betweenserving SeNB and T-MeNB.

T-MeNB transmits RRC Connection Reconfiguration message to UE whichincludes SeNB information (SeNB ID, SeNB change indicator, etc.). UEresponds with RRC Connection Reconfiguration Complete message.

Based on UE's measurement at the T-MeNB, new SeNB is selected for UEusing SeNB addition procedure.

When SeNB no longer serve UE, then the SeNB is released for UE usingSeNB release procedure.

FIG. 14 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa second embodiment of the present disclosure.

Referring to FIG. 14, in operations 14017, 1421, during MeNB handoverprocedure, new SeNB addition procedure at target MeNB can be performedwhile a procedure to maintain serving SeNB at target MeNB is performed.The new SeNB information and no change of serving SeNB indication areprovided to UE through a signaling of handover confirmation such as RRCconnection reconfiguration.

FIGS. 15A and 15B are block diagrams of a wireless communication systemaccording to a third embodiment of the present disclosure.

In the third embodiment of the present disclosure, it is assumed thatthe proposed operation is triggered in case that UE operates very lowlatency and/or ultra-reliable packet transmission/reception in higherfrequency band than current LTE.

Referring to FIG. 15A, not only serving SeNB but other SeNB(s) is alsoprepared for UE. Other SeNB(s) does not serve UE until UE switches toother SeNB(s). Other SeNB(s) for UE may be selected by MeNB based on atleast one of UE measurement including beam state information, beamindex, available resource at other SeNB(s) or stored information at MeNBof the relationship between SeNB and beam index based on UE's beamreport. A metric for SeNB switch may be determined by MeNB, where themetric may be different from that is used to send measurement report fornormal handover. A metric may be applied to beam index used or measuredbetween SeNB and UE. When UE detects signal quality degradation atserving SeNB as the metric, UE switches to one of other SeNB(s) withsending an indicator of the SeNB change to MeNB (e g, handover indicatordue to sudden link drop at serving SeNB). In other example UE switchesto one of other SeNB(s) without sending an indicator of the SeNB changeto MeNB. To support fast switching from serving SeNB to other SeNB, UEcontext is pre-fetched at other SeNB(s) under MeNB's control. To supportfast switching from serving SeNB to other SeNB, same UE context is usedamong SeNBs under same MeNB. To support fast switching from serving SeNBto other SeNB, same UE context is used among SeNBs under an eNBcontroller. UE context may include at least one of security key, MACcontext, RLC context, PDCP context. In accordance with the presentdisclosure, MeNB indicates UE which context is kept at other SeNB(s). Tosupport fast switching from serving SeNB to other SeNB, dedicated randomaccess resource at other SeNB(s) is provided to UE. In an embodiment, adedicated random access resource validity timer can be applied for thededicated random access resource. A dedicated random access resource isdetermined based on at least one of beam state information, beam indexat other SeNB(s) reported by UE.

Referring to FIG. 15B, multiple SeNB connectivity is operated withoutinvolving MeNB regardless of MeNB coexistence with SeNB. UE can beserved by SeNB as well as MeNB but the operation of this disclosure isapplied to SeNB only. In FIG. 15B, the role of MeNB is taken by servingSeNB of FIG. 15A. Other SeNB(s) are selected by serving SeNB. ServingSeNB may prepare for other SeNB(s) based on UE's measurement report,resource availability at other SeNB(s) or stored information at servingSeNB of at least one of the relationship between serving/neighboringSeNB and beam index based on UE's beam report. A metric for SeNB switchis determined by serving SeNB, where the metric may be different fromthat is used to send measurement report for normal handover. Theinformation of UE context at other SeNB(s) is provided through servingSeNB. The information of dedicated random access preamble resource maybe provided through serving SeNB. The dedicated random access preambleresource at other SeNB(s) can be selected based on beam index reportedby UE.

FIG. 16 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure.

Referring to FIG. 16, in operation 1601, UE measures adjacent MeNB andSeNB. In operation 1603, UE sends MeNB a measurement report message. Themeasurement report message may include SeNB1 information and SeNB2information. MeNB then determines active SeNBs (active set) and inactiveSeNBs (inactive set) based on the measurement report message. Inoperation 1605, UE may send an RRC connection reconfiguration message toUE. The RRC connection reconfiguration message includes selected SeNB1information, selected SeNB2 information. In operation 1607, UE and SeNB1then may start data transmission.

In the meantime, in operation 1609, in addition to serving SeNB, otherSeNB(s) can be prepared for a case where the serving SeNB becomes unableto serve UE (e.g., at least one of Radio link failure/link quality dropof serving SeNB) and service becomes unavailable through the servingSeNB before confirming UE's handover to other SeNB due to signal drop.In operation 1611, UE sends a report message including SeNB RLFindicator and new SeNB information to MeNB. The other SeNB(s) (e.g.,SeNB2) is selected based on measurement report of UE. In a possibleembodiment, the other SeNB(s) is selected based on resource availabilityat SeNB(s) for UE. In another possible embodiment, the other SeNB(s) isselected based on beam index reported by UE. MeNB store SeNB(s)information corresponding to beam state information, beam index based onUE's measurement report. SeNB addition procedure is performed betweenthe selected other SeNB(s) and MeNB. In operation 1613, the selectedother SeNB(s) is provided to UE. Based on beam index reported by UE,dedicated random access resource (e.g., at least one of preamble index,frequency resource, time resource) at other SeNB(s) is provided to UE. Ametric to switch SeNB is provided to UE, where the metric is differentfrom that to send measurement report to MeNB as handover request. Themetric is applied to measured beam (e.g., best beam or beam whose signalquality is higher than a threshold A). A metric to report SeNB switchingto MeNB is provided to UE, where the metric is different from that forsending measurement report to MeNB as handover request. The metric isapplied to measured beam (e.g., best beam or beam whose signal qualityis higher than a threshold A). UE context can be pre-fetched among MeNB,serving SeNB and other SeNB(s) based on other SeNB(s)'s location. Forexample, a SeNB is controlled by MeNB then UE context at serving SeNBcan be kept at the SeNB.

When detecting radio problem in serving SeNB as the metric, UE runs atimer which confirms serving SeNB is unavailable (e.g., the value of TTTmay be applied). After an expiration of the timer UE determines thatserving SeNB is unavailable, and reports UE's switching to other SeNB orRLF of serving SeNB to MeNB. The report may include the information ofselected other SeNB. UE switches to the newly selected SeNB. UE mayperform random access procedure with the newly selected SeNB. Ifdedicated random access resource at the newly selected SeNB is provided,UE performs access procedure using the dedicated random access resource.If it is indicated that UE context is kept at the newly selected SeNB,UE continues data transaction using current UE context.

When receiving UE's switch indicator to other SeNB, MeNB informs UE'shandover to serving SeNB and newly selected SeNB. UE's context fetchprocedure can be performed between MeNB and serving SeNB and newlyselected SeNB, if needed. Path switch procedure through newly selectedSeNB for UE can be performed if needed. MeNB informs other SeNB(s)except target SeNB to release UE's context or reserved resource for UE'sswitch.

FIG. 17 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure.

Referring to the embodiment of illustrated in FIG. 17, unlike theembodiment illustrated in FIG. 16, newly selected SeNB informs MeNB thatUE's been switched to itself without UE's switching indicator or UE'sRLF report to MeNB (in a case that operation 1611 of FIG. 16 isomitted). When UE detects the metric which is provided by MeNB is met toswitch to one of other SeNB(s), UE selects and switches to one of otherSeNB(s). Once it is indicated that UE context is kept in the newlyselected SeNB, UE continues data transaction using the current UEcontext. When target SeNB detects UE's switch through dedicated randomaccess resource or through a signaling (including identifier of UE) fromUE, target SeNB informs UE's handover to MeNB and/or serving SeNB.Target SeNB can get UE's context from MeNB and/or serving SeNB. TargetSeNB can perform path update for UE. MeNB informs other SeNB(s) excepttarget SeNB to release UE's context or reserved resource for UE'sswitch.

FIG. 18 is a flowchart illustrating a procedure of supporting handoverwith multi-connectivity in a wireless communication system according toa third embodiment of the present disclosure.

Operation 1803 of FIG. 18 sends information of SeNB2, SeNB 3.Furthermore, while in operation 1709 of FIG. 17 data is sent from UE toSeNB 1, in operation 1807 of FIG. 18 data is sent to SeNB 1 serving asMeNB from UE. Compared to UE switching to SeNB 2 in FIG. 17, UE switchesSeNB 3 in FIG. 18.

Unlike FIG. 17, in FIG. 18, serving SeNB controls a list of otherSeNB(s) which UE switch to and a metric with which UE determines toswitch to one of other SeNB(s). Serving SeNB manages UE's context(including e.g., at least one of security key, PDCP information, RLCinformation, MAC information, radio resource configuration information)which can be shared among other SeNB(s), where UE context at servingSeNB may be kept at target SeNB after UE switches to target SeNB.

In operation 1801, UE measures one or more neighboring SeNBs. Conditionsfor the measurement may include at least one of detection of a targetcell, detection of candidate cells for UE based on handover procedures,and triggering of performing handover at UE based on handoverprocedures. In operation 1803, UE sends a measurement report message toa serving SeNB. The measurement report message may include at least oneof information of SeNB2 and information of SeNB3. The serving SeNB maydetermine a list of at least one or more candidate SeNBs to which UE mayswitch based on at least one of the measurement report message receivedfrom the UE, information of resource availability of other SeNBs, beamstate information of UE, and information of candidate SeNB correspondingto UE's beam state information and beam index. Information stored for anSeNB corresponding to the beam state information and beam index may beused in providing the same candidate SeNB(s) to other UEs. In anembodiment, the serving SeNB may determine active SeNBs (active set) andinactive SenBs (inactive set) based in at least a part of informationincluded in the measurement report message.

In operation 1807, the serving SeNB sends an RRC connectionreconfiguration message to UE. In the RRC connection reconfigurationmessage, at least one of a list of candidate SeNBs, informationregarding active SeNBs, and information regarding inactive SenBs may beincluded. In an embodiment, the RRC connection reconfiguration messagemay include at least one of random access resources (including e.g., atleast one of preamble index, frequency resources, and time resources) inother SeNB(s). In an embodiment, the RRC connection reconfigurationmessage may include a metric to switch SeNB. The metric to switch SeNBis different from one to send a measurement report to SeNB as handoverrequest. The metric may be applied to measured beam (e.g., best beam orbeam whose signal quality is higher than a threshold A).

Even after the RRC connection reconfiguration message is received inoperation 1807, UE and SeNB1 may keep performing data transmission.

Meanwhile, in an embodiment, in operation 1811, the serving SeNB may beunable to serve UE (e.g., due to wireless link failure and/or linkquality drop of the serving SeNB), and due to signal drop, UE may detectthe wireless link failure with the serving SeNB. Determination of thewireless link failure may be applied to the best beam between UE and theserving SeNB. In an embodiment, once the wireless link failure isdetermined, UE runs a timer to determine that the serving SeNB isunavailable. After expiration of the timer, the UE may determine thatthe serving SeNB is unavailable. In another embodiment, UE runs a timerto determine that a link quality value of the serving SeNB is lower thanthreshold B and a link quality value of at least one of the candidateSeNBs in the list of candidate SeNBs is higher than threshold A.Determination of the link quality value may be applied to the best beambetween UE and the serving SeNB, and between UE and the at least onecandidate SeNB in the list of candidate SeNBs. After expiration of thetimer, the UE may determine that the serving SeNB is unavailable.

In operation 1813, when UE detects that a metric is met as indicated bythe serving SeNB, UE determines a target SeNB among other SeNB(s) in thelist of candidate SeNBs and switches to a target SeNB. In an embodiment,UE may perform a random access procedure with the newly selected SeNB.If a dedicated random access resource at the newly selected SeNB isprovided, UE performs an access procedure using the dedicated randomaccess resource. In an embodiment, the dedicated random access resourcemay be determined based on beam state information/beam index reported byUE to other SeNB(s). If it is indicated that UE context is kept at thenewly selected SeNB, UE continues data transaction using current UEcontext. Information indicating that the dedicated random accessresource at the selected SeNB and the UE context are kept in the newlyselected SeNB may be included in the RRC connection reconfigurationmessage.

The target SeNB recognizes the UE's switching through signaling(including an identifier of UE) from UE or the dedicated random accessresource. The target SeNB informs UE's switching to the serving SeNBand/or other SeNB(s) except itself. The target SeNB can obtain UE'scontext from the serving SeNB or designated network entity for UEcontext if needed. The target SeNB performs a path update procedure forUE.

FIGS. 19 to 21 are flowcharts illustrating procedures of SeNB additionaccording to embodiments of the present disclosure.

FIG. 19 presents MeNB initiated SeNB selection with simultaneousattachment according to an embodiment of the present disclosure.

Referring to FIG. 19, in operation 1901, UE sends an initial attachmentmessage to MeNB. The initial attachment message may include at least oneof UE capability, SeNB information. In the event that UE has to providea high rate data service in operation 1905, search for SeNB is done atUE for three SeNBs in operation 1907. After measuring, UE providesmeasurement report back to MeNB in operation 1909. In this case, UEreports SeNB1 and SeNB2 to MeNB. In this case, three SeNBs are measuredand two SeNBs (SeNB1, SeNB2) are reported to the MeNB in operation 1909.The reported information is at least one of signal quality of SeNB(s),beam state information, beam index of SeNB(s). In operations 1911 and1913, MeNB requests all possible SeNBs for addition availability. As aresponse to operations 1911 and 1913, MeNB receives a response messageto the request in operations 1915, 1917. In operation 1918, MeNBdetermines possible SeNBs based on UE's measurement report (at least oneof signal quality, beam state information, beam index). After receivingthe positive response from SeNBs which addition requests have beentransmitted to, MeNB makes information on various association levels toeach SeNB. In this case the information of association level includes atleast one of the set of SeNBs of which downlink and uplink UE should besynchronized to, the set of SeNBs of which either downlink or uplink UEshould be synchronized to, the set of SeNBs in which UE has to do thebeam training and the set of SeNBs which UE receives the data. Dedicatedrandom access resource at SeNB can be provided in the information.Maintenance information of UE context included for SeNB can be providedin the information (e.g., including whether to maintain UE context inselecting the chosen SeNB). Then MeNB send this information to UE withinRRC connection reconfiguration in operation 1919. Once UE receives thisinformation in operation 1921, it follows this configuration and itsends back to MeNB the complete message. In operations 1923, 1925, MeNBsends BS reconfiguration completion messages to selected SeNBs.According to this preference information UE tries to attach to eachSeNBs in operations 1927, 1929. The number of SeNBs which can beattached to by UE is not only 1 but the multiple SeNBs can be attachedby this procedure. FIG. 20 presents MeNB initiated SeNB selection withsequential attachment.

FIG. 20 presents MeNB initiated SeNB selection with sequentialattachment according to an embodiment of the present disclosure.

Compared to simultaneous attachment case above where MeNB makes andgives the attachable SeNB information to UE and UE attaches to themalmost simultaneously, in the sequential attachment MeNB createsattachable SeNB information sequentially. After each transfer of theSeNB information from MeNB to UE, UE attaches to that SeNB one-by-one.

UE attaches to MeNB through an initial access procedure. MeNB givesneighbor SeNB information in RRC connection setup message. At some time,HDR data is generated. Then UE attaches to and communicates with MeNBand SeNB1 after the procedure of FIG. 19. MeNB initiates SeNB selectionwith simultaneous attachment. The procedure or operation 2007 is thesame as in FIG. 19. MeNB initiated SeNB selection with simultaneousattachment section except that SeNB1 is the only SeNB to attach thistime, for example. After some time, SeNB3 has appeared due to situationchanged such as the movement of UE. Then UE measures SeNB3 in operation2009 and reports the measurement on SeNB3 in operation 2011. Inoperations 2013, 2015, BS addition request and response is transferredto/from SeNB from/to MeNB respectively. Then MeNB can give the finalattachable SeNB information to UE in RRC connection reconfiguration inoperation 2017. The final attachable SeNB is determined based on atleast one of UE's measurement report (at least one of signal quality,beam state information, beam index), resource availability at SeNB, therelationship between beam state information, beam index and SeNB. Theinformation made from MeNB may include at least one of the set of SeNBsof which downlink and uplink UE should be synchronized to, the set ofSeNBs of which either downlink or uplink UE should be synchronized to,the set of SeNBs in which UE has to do the beam training and the set ofSeNBs which UE receives the data. Dedicated random access resource atSeNB can be provided in the information. Maintenance information of UEcontext about whether to maintain the UE context of the serving SeNB atthe selected SeNB can be provided in the information. In operation 2019,UE gives RRC connection reconfiguration complete message back to MeNB.MeNB receives this complete message and sends BS reconfigurationcomplete message to SeNB3 in operation 2021. Then in operation 2023, UEattaches to SeNB3 using beamformed RACH.

FIG. 21 presents UE initiated SeNB selection with simultaneousattachment according to an embodiment of the present disclosure.

In this case, MeNB sends the measurement configuration information onSeNBs to UE. Then UE can measure and select appropriate SeNBs and accessto SeNBs with that order.

Referring to FIG. 21, in operation 2101, UE attaches to MeNB with theinformation of UE capability on SeNB operation. In operation 2103, MeNBgives the neighbor SeNB information in RRC connection reconfigurationmessage to UE. In the event that a high rate data service needs to beprovided in operation 2105, UE searches the SeNBs by measuring the SeNBsof which information is already given before, and selects theappropriate SeNBs to attach to, in operation 2107. In this case, inoperation 2109, UE selects SeNB1 and SeNB2 and sends the indicator ofthis selection to MeNB.

The selection of SeNBs to report may be based on measurement reportingcontrol sent by MeNB. In this case, three SeNBs are measured and twoSeNBs (SeNB1, SeNB2) are reported to the MeNB. Then, MeNB requests allpossible SeNBs for addition availability in operations 2111, 2113.Possible SeNBs are selected based on UE's measurement report (at leastone of signal quality, beam state information, beam index) or storedinformation of relationship between beam state information, beam indexand SeNB.

After receiving the positive response from SeNBs which addition requestshave been transmitted to in operations 2115, 2117, MeNB transmits theinformation on the SeNBs which the positive response came from withinRRC connection reconfiguration, at least one of the set of SeNBs ofwhich downlink and uplink UE should be synchronized to, the set of SeNBsof which either downlink or uplink UE should be synchronized to, the setof SeNBs in which UE has to do the beam training and the set of SeNBswhich UE receives the data etc., to UE, in operation 2119. Dedicatedrandom access resource at SeNB can be provided in the information.

Maintenance information of UE context about whether to maintain the UEcontext of the serving SeNB at the SeNB can be provided in theinformation. In this case the related information can be variable. OnceUE receives this information, it attaches to the selected SeNBs after itsends back to MeNB the complete message in operation 2121. MeNB sends BSreconfiguration completion messages to each of the selected SeNBs inoperations 2123, 2125.

In this example, UE makes the attach order SeNB1 first and SeNB2 thesecond on its own. UE first attaches to the SeNB1 which was ranked thefirst or main SeNB using beamformed RACH procedure in operation 2127.And then UE can attaches to the remaining SeNB using beamformed RACHprocedure in operation 2129.

FIG. 22 presents UE initiated SeNB selection with sequential attachmentaccording to an embodiment of the present disclosure.

Referring to FIG. 22, in operation 2201, UE perform initial attach toMeNB with UE capability of SeNB operation. In operation 2203, MeNB sendRRC connection setup message including neighbor SeNB information. HDRdata is generated at some time at UE, in operation 2205. Then UEsearches SeNBs and communicates with MeNB and SeNB1, in operation 2207.

In FIG. 22, we assume that SeNB1 is only admitted by UE measurementreport or resource availability for example. After some time, SeNB3appears so that UE measures that SeNB3 in operation 2209. Then UEdetermines this SeNB as candidate to attach to in operation 2203 and UEsends indicator including the information of SeNB3 (at least one ofSeNB3 identifier, beam state information, beam index, signal quality) tothe MeNB in operation 2205. MeNB send BS addition request to SeNB3 inoperation 2213 and SeNB3 sends back the response to the MeNB inoperation 2215.

MeNB finally gives the attachable SeNB which is SeNB3 in this example inthe RRC connection reconfiguration message to UE in operation 2217.Attachable SeNB decision is based on at least one or more of resourceavailability, signal quality, beam state information, beam index,relationship between beam state information, beam index and SeNB.Dedicated random access resource at SeNB3 can be provided. An indicationfor UE context maintenance, which provides information about whether tomaintain UE context may be provided.

In operation 2219, UE sends back the complete message in the RRCconnection reconfiguration complete message to MeNB. In operation 2221,MeNB sends BS reconfiguration complete message to SeNB3 for indicatingthat there will be a rach procedure.

Then UE attaches to SeNB3 using beamformed RACH procedure in operation2223. In the first search and attachment procedure, there is no limit ofthe number of attaching SeNBs and of course the next attachmentprocedure also has no limit on this.

FIG. 23 presents MeNB initiated multiple PCell set configurationaccording to an embodiment of the present disclosure.

Referring to FIG. 23, in operation 2301, UE performs initial connectionto MeNB. MeNB sends an RRC connection reconfiguration message includinginformation of neighboring SeNBs, in operation 2303.

In operation 2305, the UE measures cells in MeNB. After measuring, UEprovides the measurement report back to MeNB in operation 2307. In thiscase, multiple cells are measured. And MeNB determines which cell can beselected for candidate PCell in operation 2309. The candidate PCell isdefined as the cell which has a similar context as PCell for a UE butcandidate PCell may not air signaling with UE until candidate PCell ischanged to serving PCell. Then, MeNB creates information on variousassociation levels to several candidate PCells.

In this case, the information of association level may include at leastone of the set of candidate PCells of which UE should be synchronized toin uplink or downlink, the set of candidate PCells of which eitherdownlink or uplink UE should be synchronized to. Then MeNB send thisinformation to UE through RRC connection reconfiguration in operation2311.

Once UE receives this information it follows this configuration and itsends back to MeNB the complete message in operation 2313. According tothis preference information UE gets 1 or more candidate PCells which canserve as new PCell when currently serving PCell has become unable toserve the UE.

The UE may perform RACH procedure to be synchronized with the addedcandidate PCells in operation 2315. If the signal quality of currentPCell meets a condition to change PCell, (e.g., the signal quality ofcurrent PCell is less than one of candidate PCells), the UE performsPCell change from current PCell to one of candidate PCells without RRCsignaling transaction. In this case, the UE immediately may perform arandom access procedure to synchronize a new PCell, if need.

FIG. 24 presents MeNB initiated multiple PSCell set configurationaccording to an embodiment of the present disclosure.

Referring to FIG. 24, UE performs initial connection to MeNB through UEcapability of SeNB operation, in operation 2401. MeNB sends an RRCconnection reconfiguration message including information of neighboringSeNBs, in operation 2403.

In operation 2405, search for secondary cell (SCell) is done at UE for aSeNB. After measuring, UE provides the measurement report back to MeNBin operation 2407. In this case, UE sends measurement reports aboutmultiple SCells to MeNB. And MeNB sends SeNB addition request messageincluding Multiple SCell information to a SeNB in operation 2409. Themeasurement report may include signal quality, beam state information,beam index, etc. Then, SeNB determines whether notified cells can beselected or not as candidate primary secondary cell (PSCell), inoperation 2411.

The candidate PSCell is defined as the cell which has the same orsimilar context as PSCell for a UE but candidate PSCell does not do airsignaling with UE until candidate PSCell is changed to PSCell. Afterdeciding, the SeNB sends SeNB addition request acknowledge message toMeNB in operation 2413. After receiving the positive response from SeNBwhich addition requests have been transmitted to, MeNB createsinformation on various association levels to each cell.

In this case the information of association level may include at leastone of the set of candidate PSCells of which UE should be synchronizedto in downlink and uplink, the set of candidate PSCells of which eitherUE should be synchronized to in downlink or uplink, the beam-forminginformation of the set of candidate PSCells. Dedicated random accessresources for the set of candidate PSCells can be provided. Then MeNBsend this information to UE through RRC connection reconfiguration, inoperation 2415. Once UE receives this information it follows thisconfiguration and it sends back to MeNB the complete message inoperation 2417. MeNB sends SeNB reconfiguration completion messages to aSeNB in operation 2419.

According to the preference information UE gets 1 or more candidatePSCell which can serve as new PSCell when currently serving PSCell hasbecome unable to serve the UE. The UE may perform RACH procedure to besynchronized with the added candidate PSCells in operation 2421.

Also, the UE can send measurement report about multiple SeNBs to MeNB asdescribed from FIGS. 19 to 22 procedures. In this case, each SeNB hasone PSCell, several candidate PSCells and this information should besent to UE via MeNB. In this case, MeNB may select serving SeNB amongseveral SeNBs based on at least one of UE's measurement report, resourceavailability at SeNBs, stored information of relationship between beamstate information, beam index and SeNB as described procedures of FIGS.19 to 22.

If the signal quality of current PSCell meets a condition to changePSCell, (e.g., the signal quality of current PSCell is less than one ofcandidate PSCells), the UE performs PSCell change from current PSCell toone of candidate PSCells without RRC signaling transaction. In thiscase, the UE may perform a random access procedure to synchronize a newPSCell, if need.

FIG. 25 presents MeNB initiated multiple PCell set configuration duringSeNB change according to an embodiment of the present disclosure.

Referring to FIG. 25, in this case, search for SCell is done at UE for anew SeNB. The measurement control of operation 2501 is similar to thatof operation 201 shown in FIG. 2. After measuring in operation 2503, UEprovides the measurement report back to MeNB in operation 2505. In thiscase, UE sends measurement reports about multiple SCells related withnew SeNB to MeNB. The measurement reports may include at least one ofsignal quality of SCells, beam state information, beam index of SCells.In operation 2507, MeNB sends SeNB addition request message includingMultiple SCell information to a new SeNB. Then, in operation 2509, newSeNB determines whether notified cells can be selected or not forcandidate PSCell as well as PScell based on at least one of resourceavailability of SCells, UE's measurement of SCells, relationship betweenbeam state information, beam index and SCell. The candidate PSCell isdefined as the cell which has the same or similar context as PSCell fora UE but candidate PSCell may not do air signaling with UE untilcandidate PSCell is changed to PSCell.

After deciding, the new SeNB sends SeNB addition request acknowledgemessage to MeNB in operation 2511. MeNB sends an SeNB release requestmessage to S-SeNB, in operation 2513. After receiving the positiveresponse from SeNB which addition requests have been transmitted to,MeNB creates information on various association levels to each Cell, inoperation 2513.

In this case the information of association level can include at leastone of the set of candidate PSCells of which UE should be synchronizedto in downlink and uplink, the set of candidate PSCells of which eitherUE should be synchronized to in downlink or uplink, dedicated randomaccess resources of the set of candidate PSCells. Then MeNB send thisinformation to UE through RRC connection reconfiguration in operation2515. Once UE receives this information it follows this configurationand it sends back to MeNB the RRC connection reconfiguration completemessage, in operation 2517.

MeNB sends SeNB reconfiguration completion messages to a new SeNB inoperation 2519. According to this preference information UE gets 1 ormore candidate PSCells for the new SeNB, which can serve as new PSCellwhen current PSCell has become unable to serve the UE. The UE mayperform RACH procedure to be synchronized with the added candidatePSCells as well as PSCell, in operation 2521.

Also, the UE can send measurement report about multiple new SeNBs toMeNB as described procedures of FIG. 19 to FIG. 22. In this case, eachnew SeNB has one PSCell, several candidate PSCells and this informationmay be sent to UE via MeNB. In this case, MeNB may select serving newSeNB among several new SeNBs as described procedures of FIGS. 19 to 22.

If the signal quality of current PSCell meets a condition to changePSCell, (e.g., the signal quality of current PSCell is less than one ofcandidate PSCells), the UE performs PSCell change from current PSCell toone of candidate PSCells without RRC signaling transaction. In thiscase, the UE may perform a random access procedure to synchronize a newPSCell, if need.

FIG. 26 presents MeNB initiated multiple PCell set and PSCell setconfiguration during MeNB change according to an embodiment of thepresent disclosure.

Referring to FIG. 26, in this case, when a UE performs a MeNB handover,the UE also makes SeNB change. The UE may measure various cells about aserving MeNB, target MeNB, serving SeNB and target SeNB, in operation2603. If the UE wants MeNB handover in operation 2605, the UE providesmeasurement information about multiple Cells for target MeNB as well asmultiple SCells for target SeNB to a target MeNB, in operation 2607.Measurement condition or measurement reporting condition for SeNB can bedifferent from for MeNB. A value of TTT is set to ‘0’ or more.

The serving MeNB sends an HO request message to the target MeNB inoperation 2609, and the target MeNB sends an SeNB addition requestmessage to the new S-eNB in operation 2611. An HO Request Ack message inresponse to the HO request message in operation 2609 may be optionallyprocessed in operation 2619. With the measurement information aboutmultiple Cells, target MeNB determines which cell can be selected forPCell and candidate PCell in operation 2613. The candidate PCell may beindicated as the cell which has the same or similar context as PCell fora UE but candidate PCell may not do air signaling with UE untilcandidate PCell is changed to serving PCell. Then, target MeNB createsinformation on various association levels to several candidate PCells inoperation 2613.

In this case the information of association level may include at leastone of the set of candidate PCells of which downlink and uplink UEshould be synchronized to, the set of candidate PCells of which eitherUE should be synchronized to in downlink or uplink. Also, target MeNBtransfers measurement information reported by UE about multiple SCellsfor target SeNB. The measurement information includes at least one ofsignal quality, beam state information, beam index. Then, in operation2615, target SeNB determines whether notified cells can be selected ornot for PSCell and candidate PSCell based on at least one of themeasurement information, resource availability of cells in target SeNB,stored information of relationship between beam state information, beamindex and cells in target SeNB. The candidate PSCell may be indicated asthe cell which has the same or a similar context as PSCell for a UE butcandidate PSCell may not do air signaling with UE until candidate PSCellis changed to PSCell. After deciding, target SeNB sends SeNB additionrequest acknowledge message to target MeNB in operation 2617.

The SeNB addition request acknowledge message can include dedicatedrandom access resources of selected cells at target SeNB. Afterreceiving the positive response from target SeNB which addition requestshave been transmitted to, target MeNB creates information on variousassociation levels to each SCell. In this case the information ofassociation level may include at least one of the set of candidatePSCells of which UE should be synchronized to in downlink and uplink,the set of candidate PSCells of which either UE should be synchronizedto in downlink and uplink, beam-forming information of the set ofcandidate PSCells, dedicated random access resource of the set ofcandidate PSCells which resource is determined based on beam stateinformation, beam index.

Then target MeNB send this information to serving MeNB and the MeNBtransmits this information to UE through RRC connection reconfigurationin operation 2621. Once UE receives this information it follows thisconfiguration and it sends back to MeNB the RRC connectionreconfiguration complete message. According to this preferenceinformation UE tries to attach to PCell and PScell, and the UE gets 1 ormore candidate PCells, and 1 or more candidate PSCells. The candidatePCell can serve as new PCell when current PCell has become unable toserve the UE. The candidate PScell can serve as new PScell when currentPScell has become unable to serve the UE. In operations 2633, 2639, theUE may perform RACH procedure to be synchronized with the addedcandidate PSCells and the added candidate PCells.

Also, the UE can send measurement report about multiple new SeNBs toMeNB as described procedures of FIGS. 19 to 22. In this case, each newSeNB has one PSCell, several candidate PSCells and this informationshould be sent to UE via MeNB. In this case, MeNB may select new servingSeNB among several new SeNBs as described procedures in FIGS. 19 to 22.If the signal quality of current PCell meets a condition to changePCell, (e.g., the signal quality of current PCell is less than one ofcandidate PCells), the UE may perform PCell change from current PCell toone of candidate PCells without RRC signaling transaction.

In this case, the UE may perform a random access procedure tosynchronize a new PCell, if need. In similar way PSCell change fromcurrent PScell to one of candidate PScells may be performed without RRCsignaling transaction if PSCell change condition is met. A condition tochange PSCell can be different from that of PCell and it may be appliedto beam index of PSCell.

In an embodiment of the present disclosure, MeNB manages a serving celland a list of candidate cells based on the measurement report from theUE, and a candidate cell may be selected for a cell to which the UE isable to hand over. In this regard, handover triggering conditions andthe cell list may be determined by the serving cell.

Furthermore, once handover is determined, the UE performs inter-cellresource fetching and path switching. In this regard, the UE may send anindicator to indicate that the handover is caused from a link drop, toMeNB.

Meanwhile, additional conditions in which the UE may perform fasthandover to a candidate cell in accordance with an embodiment of thepresent disclosure may be one of the followings:

Dedicated RACH preamble resources are operated, and for the dedicatedRACH preamble resources, preamble valid timer may be operated;

RACH transmit beam information or transmit/receive beam information onthe candidate cell is secured in advance in determining the candidatecell;

UE context that has been used in the serving cell is used intact withoutrelease/reconfiguration of the UE context, and a context maintenanceindicator is operated;

In determining a candidate cell, beam index and beam state informationfor candidate cells stored in serving cell are used.

FIG. 27 is a flowchart of operation of UE in a wireless communicationsystem according to an embodiment of the present disclosure.

Referring to FIG. 27, the UE measures a serving cell and neighboringcells, in operation 2701.

The UE sends a measurement report message that reflects the measurementresults to MeNB or SeNB, in operation 2703.

The UE then receives an RRC connection reconfiguration message from MeNBor SeNB, in operation 2705. The RRC connection reconfiguration messageincludes a list of candidate cells including cell IDs.

In an embodiment, the UE determines whether a wireless link failure (orhandover) (or signal error) has occurred in SeNB, in operation 2707. Ifa wireless link failure has occurred in SeNB, the UE performs a randomaccess procedure with a new SeNB, in operation 2709. Informationrequired to perform the random access procedure with the new SeNB isobtained from MeNB or SeNB.

In an embodiment, UE may determine whether a wireless link failure (orhandover) (or signal error) with MeNB, in operation 2707. If a wirelesslink failure has occurred with MeNB, the UE performs a random accessprocedure with a new MeNB, in operation 2709. Information required toperform the random access procedure with the new MeNB may be obtainedfrom MeNB or SeNB.

In another embodiment, if any wireless link failure has not occurred inMeNB or SeNB, the UE performs the random access procedure with theexisting SeNB, in operation 2711.

Operations 2701 to 2711 of UE as shown in FIG. 27 are only by way ofexamples, and the UE accesses MeNB or SeNB and performs a handoverprocedure according to what are described in FIGS. 1 to 3, 4A, 4B, 4C, 5to 14, 15A, 15B, and 16 to 26.

FIG. 28 is a flowchart of operation of MeNB in a wireless communicationsystem according to an embodiment of the present disclosure.

In operation 2801, MeNB receives a measurement report message based onmeasurement results from the UE.

In operation 2803, MeNB determines active inactive sets based on themeasurement report message.

In operation 2805, MeNB sends an RRC connection reconfiguration messageincluding at least one of the active and inactive sets to the UE. In anembodiment, MeNB may provide various information for the UE to performthe random access procedure.

In operation 2807, MeNB determines whether a wireless link failure orsignal quality drop has occurred in SeNB. If a wireless link failure orsignal quality drop has occurred in SeNB, MeNB receives a signal errorreport message from the UE, in operation 2809.

Operations 2801 to 2809 of MeNB as shown in FIG. 28 are only by way ofexamples, and the MeNB accesses the UE and supports handover withmulti-connectivity according to what are described in FIGS. 1 to 3, 4A,4B, 4C, 5 to 14, 15A, 15B, and 16 to 26.

FIG. 29 is a flowchart of operation of SeNB in a wireless communicationsystem according to an embodiment of the present disclosure.

Referring to FIG. 29, in operation 2901, SeNB determines whether awireless link failure or signal quality drop has occurred in the SeNB.If a wireless link failure or signal quality drop has occurred in theSeNB, the SeNB performs a random access procedure with the UE, inoperation 2903.

Operations 2901 to 2903 of SeNB as shown in FIG. 29 are only by way ofexamples, and the SeNB accesses the UE and supports handover withmulti-connectivity according to what are described in FIGS. 1 to 26.

FIG. 30 is a block diagram of a UE in a wireless communication systemaccording to an embodiment of the present disclosure.

Referring to FIG. 30, a UE includes a transmitter 3000, a receiver 3010,and a controller 3020. The controller 3020 includes at least oneprocessor.

The transmitter 3000 and the receiver 3010 include a transmitting moduleand a receiving module for transmitting and receiving data to and from abase station, respectively, according to an embodiment of the presentdisclosure.

The controller 3020 accesses an MeNB or an SeNB and performs a handoverprocedure, according to what are described in connection with FIGS. 1 to4C, 5 to 15B, and 16 to 26.

The controller 3020 obtains candidate SeNBs in advance using measurementby the MeNB in preparation for a possible situation (e.g., signalquality drop) in which a problem arises in radio link with the S-SeNB.If the problem arises in the radio link with the S-SeNB, the controller3020 switches to a new S-SeNB among the candidate SenBs.

FIG. 31 is a block diagram of an MeNB in a wireless communication systemaccording to an embodiment of the present disclosure.

Referring to FIG. 31, an MeNB includes a transmitter 3100, a receiver3110, and a controller 3120.

The transmitter 3100 and the receiver 3110 include a transmitting moduleand a receiving module for transmitting and receiving data to and from aUE, respectively, according to an embodiment of the present disclosure.

The controller 3120 accesses the UE and supports handover withmulti-connectivity, according to what are described in connection withFIGS. 1 to 3, 4A, 4B, 4C, 5 to 14, 15A, 15B, and 16 to 26.

When receiving an indicator of switching of UE to another SeNB, thecontroller 3120 informs a serving SeNB and a newly selected SeNB of theUE's handover. A procedure of fetching the UE's context may be performedbetween the MeNB and the serving SeNB or the newly selected SeNB, ifnecessary. A procedure of path switching via the newly selected SeNB maybe performed for the UE, if necessary. The controller 3120 informs otherSeNB(s) except for the target SeNB to release resources reserved for theUE's switching, or the UE's context.

FIG. 32 is a block diagram of an SeNB in a wireless communication systemaccording to an embodiment of the present disclosure.

Referring to FIG. 32, an SeNB includes a transmitter 3200, a receiver3210, and a controller 3220.

The transmitter 3200 and the receiver 3210 include a transmitting moduleand a receiving module for transmitting and receiving data to and from aUE, respectively, according to an embodiment of the present disclosure.

The controller 3220 accesses the UE and supports handover withmulti-connectivity, according to what are described in connection withFIGS. 1 to 3, 4A, 4B, 4C, 5 to 14, 15A, 15B, and 16 to 26.

It should be noted that the illustrations, signal flow charts, blockdiagrams, etc. as shown in FIGS. 1 to 3, 4A, 4B, 4C, 5 to 14, 15A, 15B,and 16 to 26 are not intended to limit the scope of the presentdisclosure. Thus, all the information, fields, components, or operationsin connection with FIGS. 1 to 3, 4A, 4B, 4C, 5 to 14, 15A, 15B, and 16to 26 should not be interpreted as essential elements for practicing thepresent disclosure, and the present disclosure may be implemented onlywith a part of the components or operations within the scope of thepresent disclosure.

The aforementioned operations may be implemented by a memory devicestoring the corresponding program codes equipped in an entity, afunction, a base station of a communication system, or an arbitrary partin a vehicle. A controller of the entity, function, base station, UE, orvehicle may execute the aforementioned operations by having a processoror central processing unit (CPU) read out and process the program codesstored in the memory device.

The aforementioned entities, functions, base stations (eNBs), UEs, orvarious parts of vehicle, modules, etc., may be implemented in hardware,e.g., complementary metal oxide semiconductor based logic circuits,firmware, software, and/or a combination of hardware, firmware and/ormachine-readable software. For example, various electric structures andmethods may be implemented with electric circuits, such as transistors,logic gates, application-specific integrated circuits (ASICs), etc.

While the present disclosure has been shown and described with referenceto various embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present disclosuresdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for supporting handover of a userequipment (UE) in a wireless communication system, the methodcomprising: receiving, from a serving cell, a radio resource control(RRC) message; identifying whether the RRC message includes aconditional handover configuration, wherein the conditional handoverconfiguration includes first information on at least one targetcandidate cell and second information on condition of conditionalhandover execution; in case that the RRC message includes theconditional handover configuration, selecting one target cell among theat least one target candidate cell based on the conditional handoverconfiguration; and performing handover to the selected one target cell.2. The method of claim 1, wherein the second information is associatedwith the first information.
 3. The method of claim 1, the performingcomprising: performing a random access procedure with the selected onetarget cell.
 4. The method of claim 1, further comprising: receiving,from the serving cell, information related to a measurementconfiguration; and performing measurement on the serving cell and othercell based on the information related to the measurement configuration.5. The method of claim 4, further comprising: transmitting, to theserving cell, a measurement report message including information relatedto at least one of the serving cell or the other cell.
 6. A userequipment (UE) for supporting handover in a wireless communicationsystem, the UE comprising: a transceiver; and a controller configured tocontrol the transceiver, wherein the controller is configured to:receive, from a serving cell, a radio resource control (RRC) message,identifying whether the RRC message includes a conditional handoverconfiguration, wherein the conditional handover configuration includesfirst information on at least one target candidate cell and secondinformation on condition of conditional handover execution, in case thatthe RRC message includes the conditional handover configuration, selectone target cell among the at least one target candidate cell based onthe conditional handover configuration, and perform handover to theselected one target cell.
 7. The UE of claim 6, wherein the secondinformation is associated with the first information.
 8. The UE of claim6, wherein the controller is further configured to: perform a randomaccess procedure with the selected one target cell.
 9. The UE of claim6, wherein the controller is further configured to: control thetransceiver to receive, from the serving cell, information related to ameasurement configuration, and perform measurement on the serving celland other cell based on the information related to the measurementconfiguration.
 10. The UE of claim 9, wherein the controller is furtherconfigured to: control the transceiver to transmit, to the serving cell,a measurement report message including information related to at leastone of the serving cell or the other cell.
 11. A method for supportinghandover at a serving cell in a wireless communication system, themethod comprising: transmitting, to a user equipment (UE), a radioresource control (RRC) message including a conditional handoverconfiguration, wherein the conditional handover configuration includesfirst information on at least one target candidate cell and secondinformation on condition of conditional handover execution.
 12. Themethod of claim 11, wherein the second information is associated withthe first information.
 13. The method of claim 11, further comprising:transmitting, to the UE, information related to a measurementconfiguration; and receiving, from the UE, a measurement report messageincluding information related to at least one of the serving cell or theother cell, wherein the measurement report message includes measurementresult on the serving cell and the other cell measured based on theinformation related to the measurement configuration.
 14. A serving cellfor supporting handover in a wireless communication system, the servingcell comprising: a transceiver; and a controller configured to controlthe transceiver, wherein the controller is configured to: control thetransceiver to transmit, to a user equipment (UE), a radio resourcecontrol (RRC) message including a conditional handover configuration,and wherein the conditional handover configuration includes firstinformation on at least one target candidate cell and second informationon condition of conditional handover execution.
 15. The serving cell ofclaim 14, wherein the second information is associated with the firstinformation.
 16. The serving cell of claim 14, wherein the controller isfurther configured to: control the transceiver to transmit, to the UE,information related to a measurement configuration, and control thetransceiver to receive, from the UE, a measurement report messageincluding information related to at least one of the serving cell or theother cell, wherein the measurement report message includes measurementresult on the serving cell and the other cell measured based on theinformation related to the measurement configuration.